This invention relates to orthopedic surgery and, more specifically, to the removal of bone cement in the repair or replacement of orthopedic prosthesis.
It is common practice in orthopedic surgery, particularly in the replacement of hip joints with prosthetic devices, to fix the anchor portion of the prosthetic device or devices in the bone using one or more types of bone cement. The most popular kind of cement currently in use is a methylmethacrylate based cement which can be mixed with the catalyst immediately before use and injected or placed around the prosthetic anchor in the bone where it quickly polymerizes and hardens to form a firm bond to the bone and to the implant. One such cement is sold by Dow Corning Wright, Arlington, Tenn. 38002, under the trademark CMW 3 the bone cement is sold in two packages, one consisting essentially of methylmethacrylate monomer, which contains stabilizers. The second portion is a mixture of polymethylmethacrylate powder and barium sulfate that contains a catalyst, or initiator, benzoyl peroxide. Barium sulfate is included as a radiopaque material so that the precise configuration of the cement, after it has set in the bone, can be determined by X-ray.
When the surgical procedure has proceeded to the point where the prostheses can be fixed in place in the bone, the first portion, which is a liquid, is mixed with the second portion. The second portion is in powder form and including small particles of polymethylmethacrylate which form nucleation sites for polymerization. The second portion also contains benzoyl peroxide which initiates polymerization quite rapidly. Once the mixing is complete, the surgeon has less than ten minutes in which to complete the use of the cement in the surgical procedure before it sets. Curing continues for several hours after set up, but the bone cement sets to a solid firm configuration in about ten to fifteen minutes. Once the cement is set, the surgical procedure can be completed while the curing of the cement continues.
Hot melt adhesives, i.e. thermal plastic materials which melt upon being warmed and harden upon cooling, may also be used, see Hayakawa, Y., et al, J. Takeda Res. Lab. (Japan), 1982, 41/1-2 (72-80).
The nature of the bone cement is of not critical to the present invention, except that it must be thermoplastic, that is, when the cement is set in place it must melt or soften upon the application of heat. Nearly all such cements are based upon the monomer methylmethacrylate, but any other meltable cement is subject to the method described hereinafter and can be removed using the apparatus which is described.
It is quite common that a prostheses must be removed after some period of use. Removal may be necessary because the prostheses has loosened, because of additional or separate injury, weakening of the bone in the proximity of the cement, or any of many other conditions. The removal of cement from a previously repaired joint or from around a previously installed prostheses presents a challenging problem to the surgeon. It is important that the cement be removed quickly, so as not to extend the time of the patient on the operating table to minimize trauma and lingering discomfort to the patient and discomfort to the surgical team.
The use of heated instruments to remove bone cement has been known for many years. Tarabichy, S. et al, U.S. Pat. No. 4,702,236, Oct. 27, 1987, describes several techniques for removing bone cement, including a method using a heated blade instrument for removing the bone cement. While the use of the heated blade obviates some of the problems inherent in prior art devices and methods, it still suffers from a number of disadvantages. Since it takes a considerable period of time to scrape even the melted cement from the bone using the knife blades of Tarabichy et al, the bone and adjacent tissue of the patient is heated considerably and it is possible to damage the bone and or the tissue upon prolonged heating. In addition, the heating of the cement generates noxious fumes which make it difficult for the operating team to perform. The fumes are lacrimators and may appear in the form of smoke which obscures the surgeons view.
Huebsch, D. L., U.S. Pat. Nos. 4,873,969, Oct. 17, 1989 and 5,064,462, Nov. 12, 1991 describes uses of a thermal chisel to remove bone cement. The thermal chisel described in Huebsch are internally heated by an electric heater. According to the Huebsch patent, the heated tip is inserted into the cement to mold one or more grooves in the cement. The heated tip is removed and the cement is allowed to reharden. Then the surgeon uses an impact chisel to break away the fragments of cement. The Huebsch method and apparatus suffers in the main from the same disadvantages that were noted with respect to the Tarabichy method; namely, the necessity for introducing large quantities of heat into the cement, because of the mass of the tools used and the nature and method in which they are used, the risk of overheating the bone and surrounding tissue and the length of time it takes to heat the tools and use them in the bone.
It is a principal feature of this invention to provide a method and an apparatus which can melt the bone cement to permit insertion of a retraction tool into the cement, allowing the retraction tool to cool in the cement and then extracting the tool and the cement.
Another object of this invention is to provide a disposable-self contained bone cement retraction tool which can be pre-sterilized and packaged so that no additional sterilization or further steps are required before use by the surgeon.
Another feature of the invention is the provision of an instrument which requires minimal amounts of energy to be introduced into the bone to obviate the risks of damage to the bone and surrounding tissues and the introduction of noxious fumes and smoke into the operating room.